Several different cumulus parameterizations are compared in a 10-day
regional model simulation over the tropical Americas in northern summer.
A simple bulk diagnostic test is devised, comparing the model's preferred
domain-mean wind divergence profile with °»observed°… drivergence. The
latter is obtained by a line integral of the normal wind component at the
model's outer boundary, from the ECMWF reanalysis data used as lateral
boundary conditions. The former is obtained from a line integral one grid
point in from the boundary, a perimeter that encloses almost exactly the
same region. Even though the model fields near the boundary are strongly
nudged toward the ECMWF values, the difference is distinct, and indicative
of systematic errors in the model's heating field throughout the interior
of the domain. Heating reflects the effects of the convection scheme, both
direct and indirect (e.g., through its impact on resolved condensation). A
useful axis along which to characterize schemes appears to be overactive
versus underactive. Underactive convective schemes tend to produce too
little low-level convergence and upper-level divergence, while overactive
schemes produce too much. This categorization is also reflected in rainfall
fields, as overactive schemes produce widespread light convective rain
while underactive schemes produce sparse occasional storms. For example,
the Kain-Fritsch scheme is overactive with its default entraining-plume
radius of 1500 m, a value optimized for midlatitudes over land. A value of
750 m makes the regional divergence magnitude about right, but makes the
upper-tropospheric outflow altitude too low, illustrating a classic dilemma
of entraining-plume models of convection. Schemes with other conceptual
structures give widely varying divergence errors. The largest errors are
found with the Anthes-Kuo scheme, while the smallest errors are found with
the Betts-Miller-Janjic scheme, which has no consistent divergence bias
over time. Diagnosis of other North American monsoon simulations supports
the general underactive/overactive characterization, but shows that the
best scheme and parameters may depend on weather regime.